In the conventional magnetic disk device, a dusttight sealing device is used in a space between a shaft and a housing to seal its inside from an outside environment.
This sealing device is comprised of a pair of pole pieces and an annular permanent magnet sandwiched therebetween. There is a gap between the pole pieces and the shaft to hold a ferrofluid in the gap. In the sealing device, the pole pieces are directly secured to the housing which is of magnetic material, so that there is a magnetic flux path extending through one of the pole pieces, the housing and the other of the pole pieces. Consequently, the magnetic flux circuit to hold the ferrofluid is short-circuited by the magnetic flux path through the pole pieces and the housing, and the greater part of the magnetic flux from the permanent magnet flows into the housing.
For this reason, the density of the magnetic flux in the gap between the shaft and the pole pieces decreases, and the force maintaining the ferrofluid becomes weak. This gives rise to the problem of a striking decrease in the pressure resistance of the ferrofluid.
It is believed that this problem can be eliminated if the housing is constructed of a nonmagnetic material such as aluminum. However, this is no effective strategic solution for an antifriction bearing, in which the housing must be a magnetic material and could not be made from a nonmagnetic material.
Under these circumstances, a strategic solution has already been proposed with the premise that the housing is a magnetic body. This is the sealing device shown in FIG. 1, which shows a housing 1 made of magnetic body, a shaft 3 made of the same magnetic body, and a ferrofluid seal device 5, and a ring spacer 7 made from a nonmagnetic body interposed between the ferrofluid seal device 5 and the housing 1. The ferrofluid seal device 5 comprises an annular magnet 11 magnetized in the axial direction, and a pair of annular pole pieces 13 and 15 between which the magnet 11 is integrally inserted. A gap g is provided between the shaft 3 and the pole pieces 13 and 15 secured to the housing 1 through the ring spacer 7. A ferrofluid 9 made from an oil containing fine magnetic particles in the order of 0.001 .mu.m in size is retained in the gap g by the magnetic field generated by the magnet 11.
However, in a sealing device with this type of configuration, not only the ferrofluid seal device 5 but also the ring spacer 7 must be incorporated in the housing 1, so that parts and material costs are high.
In addition, because the ring spacer 7 is interposed between the housing 1 and the ferrofluid seal device 5, an additional engagement section is required, so that the positional precision of the pole pieces 13 and 15 with respect to the shaft 3 is worsened. Specifically, a dimensional deviation is produced in the gap g around the shaft 3.
In addition, in the case of applying the ferrofluid seal device to the bearing assembly of the magnetic disk device as shown in FIG. 2, which has an inner race 17, an outer race 19 and balls 21, because the spacing between the inner race 17 and the outer race 19 is about 2 mm, which is extremely narrow, it would be difficult to install the seal device 5 and the ring spacer 7 with good precision between the inner race 17 and the outer race 19 so as to seal the lubricated section of a ball 21. If a desired precision is to be obtained, it is necessary to improve the precision of the processing of the pole pieces 13 and 15 and the ring spacer 7, which increases the cost of production.